1. Technical Field
The embodiments described herein relate to a phase-change random access memory device, and in particular, to a phase-change random access memory device that is capable of minimizing the size of a bottom electrode contact, and a method of manufacturing the same.
2. Related Art
Phase-change Random Access Memory (PRAM) devices have been developed in part to overcome certain limitations of existing memory devices, such as Dynamic Random Access Memories (DRAMs), Static Random Access Memories (SRAMs), and flash memories.
A PRAM is a memory device that writes and reads out information on the basis of a reversible phase change of a phase-change material, which has high resistance in an amorphous state and low resistance in a crystalline state. A PRAM device can provide higher operational speed and a higher degree of integration than, e.g., a conventional flash memory device.
FIG. 1 is a graph illustrating the operation principle of a PRAM device.
As shown in FIG. 1, when the phase-change material layer is heated at a temperature higher than a melting temperature Tm for a short time (first operation period: t1), and then cooled at high speed as represented by curve (A), a phase-change material layer is changed to the amorphous state. In contrast, when the phase-change material layer is heated at a temperature lower than the melting temperature Tm and higher than a crystallization temperature Tc for a time (second operation period: t2) longer than the first operation period t1, and then cooled as represented by curve (B), then the phase-change material layer is changed to the crystalline state.
Here, the resistivity of the phase-change material layer in the amorphous state is higher than the resistivity of the phase-change material layer in the crystalline state. Accordingly, in a read mode, it is possible to determine whether or not the information stored in the PRAM device is a logic level ‘1’ or a logic level ‘0’ by detecting a current flowing in the phase-change material layer.
As such, when a high-density current flows through a contact area to the phase-change material layer of the PRAM device, the crystalline state of the contact surface of the phase-change material layer is changed. As the contact area becomes small, the density of a current required for changing the state of the phase-change material can be reduced. To this end, a method that forms a bottom electrode contact in a plug shape has been introduced.
FIG. 2 is a diagram illustrating the structure of a conventional PRAM device having a plug-shaped bottom electrode contact.
As shown in FIG. 2, an interlayer insulating film 3 is formed on a semiconductor substrate 1, on which a bottom electrode 2 is formed, and a hard mask 4 is formed on the interlayer insulating film 3. Next, a cylindrical bottom electrode contact hole is formed by exposure and etching.
Subsequently, a metal that generates Joule heat required for phase change is filled into the bottom electrode contact hole to form a bottom electrode contact (BEC) 5. Next, a phase-change material layer (not shown) and a top electrode (not shown) are sequentially formed.
In a conventional PRAM device, however, since the contact area of a phase-change material layer pattern and the BEC entirely depends on photolithography and etching on the BEC hole, there is a difficulty in reducing the contact area. Accordingly, the amount of a current required for phase change is increased, which makes it difficult to achieve low power consumption and high integration. In addition, a variation occurs in the area of the BEC, which is formed by photolithography and etching, which results in deterioration of the reliability of the memory device.